Long-term monitoring in IC4665: Fast rotation and weak variability in very low mass objects

We present the combined results of three photometric monitoring campaigns targeting very low mass (VLM) stars and brown dwarfs in the young open cluster IC4665 (age ~40 Myr). In all three runs, we observe ~100 cluster members, allowing us for the first time to put limits on the evolution of spots and magnetic activity in fully convective objects on timescales of a few years. For 20 objects covering masses from 0.05 to 0.5 Msol we detect a periodic flux modulation, indicating the presence of magnetic spots co-rotating with the objects. The detection rate of photometric periods (~20%) is significantly lower than in solar-mass stars at the same age, which points to a mass dependence in the spot properties. With two exceptions, none of the objects exhibit variability and thus spot activity in more than one season. This is contrary to what is seen in solar-mass stars and indicates that spot configurations capable of producing photometric modulations occur relatively rarely and are transient in VLM objects. The rotation periods derived in this paper range from 3 to 30h, arguing for a lack of slow rotators among VLM objects. The periods fit into a rotational evolution scenario with pre-main sequence contraction and moderate (40-50%) angular momentum losses due to wind braking. By combining our findings with literature results, we identify two regimes of rotational and magnetic properties, called C- and I-sequence. Main properties on the C-sequence are fast rotation, weak wind braking, Halpha emission, and saturated activity levels, while the I-sequence is characterised by slow rotation, strong wind braking, no Halpha emission, and linear activity-rotation relationship. Rotation rate and stellar mass are the primary parameters that determine in which regime an object is found. (abridged)Comment: 16 pages, 10 figures, accepted for publication in MNRA

Wer das Geld hat, hat die Macht? : Verhandlungen von Property Gaps in Paarwelten

Während für Paare eine zunehmende Verbreitung partnerschaftlicher Beziehungsnormen konstatiert wird, herrschen nach wie vor Geschlechterungleichheiten, die sich u.a. im gender wage gap oder gender care gap dokumentieren. Entsprechend wurden bislang Ungleichheiten bei Paaren primär anhand von Einkommensdifferenzen oder entlang der innerpartnerschaftlichen Verteilung von bezahlter Erwerbs- und unbezahlter Fürsorgearbeit untersucht. Es fehlen Analysen zu den ungleichheits(re)produzierenden Mechanismen der Eigentumsverteilung und -aushandlung innerhalb von Paarbeziehungen, denn Eigentum geht über Einkommen hinaus und umfasst Vermögen, Immobilien, Sachgüter, aber auch Schulden als Form negativen Eigentums. Der Beitrag fokussiert ausgehend von Interviewmaterial aus dem Teilprojekt B06 des SFB/TR 294 "Strukturwandel des Eigentums" auf den paarinternen Umgang mit ungleichen Eigentumsausstattungen (property gaps). Mittels eines Fallvergleichs werden Strategien und Effekte der Umverteilung und Vergemeinschaftung von Eigentum zur Überbrückung von property gaps reflektiert. Es zeigt sich, dass in der Analyse des paarinternen doing property die Differenzierung zwischen Eigentumsströmen (flows) und Eigentumsbeständen (stocks) Berücksichtigung finden muss. Zudem fungiert der individuelle Erfahrungshintergrund der Klassenherkunft – neben Geschlecht – als Regulativ bei den meist subtil ablaufenden Eigentumsverhandlungen in Paarbeziehungen. Insgesamt wird für eine stärkere Berücksichtigung der Eigentumsdimension in der Geschlechterforschung plädiert

About the nature of Mercer14

We used UKIRT near infrared (NIR) broad band JHK photometry, narrow band imaging of the 1-0S(1) molecular hydrogen emission line and mid infrared Spitzer IRAC data to investigate the nature of the young cluster Mercer14. Foreground star counts in decontaminated NIR photometry and a comparison with the Besancon Galaxy Model are performed to estimate the cluster distance. This method yields a distance of 2.5kpc with an uncertainty of about 10% and can be applied to other young and embedded clusters. Mercer14 shows clear signs of ongoing star formation with several detected molecular hydrogen outflows, a high fraction of infrared excess sources and an association to a small gas and dust cloud. Hence, the cluster is less than 4Myrs old and has a line of sight extinction of A_K=0.8mag. Based on the most massive cluster members we find that Mercer14 is an intermediate mass cluster with about 500Mo.Comment: 10pages, 2tables, 5figures, accepted for publication by MNRAS, a version with higher resolution figures can be found at http://astro.kent.ac.uk/~df

Rotation and variability of very low mass stars and brown dwarfs near Epsilon Ori

We explore the rotation and activity of very low mass (VLM) objects by means of a photometric variability study. Our targets in the vicinity of Epsilon Ori belong to the OriOB1b population in the Orion star-forming complex. In this region we selected 143 VLM stars and brown dwarfs (BDs), whose photometry in RIJHK is consistent with membership of the young population. The variability of these objects was investigated using a densely sampled I-band time series covering four consecutive nights with altogether 129 data points per object. Our targets show three types of variability: Thirty objects, including nine BDs, show significant photometric periods, ranging from 4h up to 100h, which we interpret as the rotation periods. Five objects, including two BDs, exhibit variability with high amplitudes up to 1 mag which is at least partly irregular. This behaviour is most likely caused by ongoing accretion and confirms that VLM objects undergo a T Tauri phase similar to solar-mass stars. Finally, one VLM star shows a strong flare event of 0.3 mag amplitude. The rotation periods show dependence on mass, i.e. the average period decreases with decreasing object mass, consistent with previously found mass-period relationships in younger and older clusters. The period distribution of BDs extends down to the breakup period, where centrifugal and gravitational forces are balanced. Combining our BD periods with literature data, we found that the lower period limit for substellar objects lies between 2h and 4h, more or less independent of age. Contrary to stars, these fast rotating BDs seem to evolve at constant rotation period from ages of 3 Myr to 1 Gyr, in spite of the contraction process. Thus, they should experience strong rotational braking.Comment: 17 pages, 13 figures, A&A, in pres

Impact of Record-Linkage Errors in Covid-19 Vaccine-Safety Analyses using German Health-Care Data: A Simulation Study

With unprecedented speed, 192,248,678 doses of Covid-19 vaccines were administered in Germany by July 11, 2023 to combat the pandemic. Limitations of clinical trials imply that the safety profile of these vaccines is not fully known before marketing. However, routine health-care data can help address these issues. Despite the high proportion of insured people, the analysis of vaccination-related data is challenging in Germany. Generally, the Covid-19 vaccination status and other health-care data are stored in separate databases, without persistent and database-independent person identifiers. Error-prone record-linkage techniques must be used to merge these databases. Our aim was to quantify the impact of record-linkage errors on the power and bias of different analysis methods designed to assess Covid-19 vaccine safety when using German health-care data with a Monte-Carlo simulation study. We used a discrete-time simulation and empirical data to generate realistic data with varying amounts of record-linkage errors. Afterwards, we analysed this data using a Cox model and the self-controlled case series (SCCS) method. Realistic proportions of random linkage errors only had little effect on the power of either method. The SCCS method produced unbiased results even with a high percentage of linkage errors, while the Cox model underestimated the true effect

Very low-mass members of the Lupus 3 cloud

We report on a multi-band survey for very low-mass stars and brown dwarfs in the Lupus 3 cloud with the Wide Field Imager (WFI) at the ESO/MPG 2.2m telescope on La Silla Observatory (Chile). Our multiband optical photometry is combined with available 2MASS JHK photometry to identify 19 new young stars and 3 brown dwarf candidates as probable members of this star forming region. Our objects are mostly clustered around the cloud core. Stars and brown dwarfs have similar levels of H-alpha emission, probably a signature of accretion. One object, a brown dwarf candidate, exhibits a near-infrared excess, which may indicate the presence of a disk, but its H-alpha emission cannot be confirmed due to its faintness in the optical passbands. We also find two visual pairs of probable Lupus 3 members that may be wide binaries

SSPMJ1102-3431 brown dwarf characterization from accurate proper motion and trigonometric parallax

In 2005, Scholz and collaborators (Scholz et al. 2005) discovered, in a proper motion survey, a young brown dwarf SSSPMJ1102-3431(SSSPMJ1102) of spectral type M8.5, probable member of the TW Hydrae Association (TWA) and possible companion of the T Tauri star TW Hya. The physical characterization of SSSPMJ1102 was based on the hypothesis that it forms a binary system with TW Hya. The recent discovery of a probable giant planet inside the TW Hya protoplanetary disk with a very short-period (Setiawan et al. 2008) and a disk around SSSPMJ1102 (Riaz and Gizis 2008) make it especially interesting and important to measure well the physical parameters of SSSPMJ1102. Trigonometric parallax and proper motion measurements of SSSPMJ1102 are necessary to test for TWA membership and, thus, to determine the mass and age of this young brown dwarf and the possibility that it forms a wide binary system with TW Hya. Two years of regular observations at the ESO NTT/SUSI2 telescope, have enabled us to determine the trigonometric parallax and proper motion of SSSPMJ1102. Our parallax and proper motion determination allow us to precisely describe the physical properties of this low mass object and to confirm its TWA membership. Our results are not incompatible with the hypothesis that SSSPMJ1102 is a binary companion of the star TW Hya.Comment: accepted for publication in A&

Rotation and accretion of very low mass objects in the SigmaOri cluster

We report on two photometric monitoring campaigns of Very Low Mass (VLM) objects in the young open cluster around SigmaOrionis. Our targets were pre-selected with multi-filter photometry in a field of 0.36 sqdeg. For 23 of these objects, spanning a mass range from 0.03 to 0.7 MS, we detect periodic variability. Of these, 16 exhibit low-level variability, with amplitudes of less than 0.2 mag in the I-band, which is mostly well-approximated by a sine wave. These periodicities are probably caused by photospheric spots co-rotating with the objects. In contrast, the remaining variable targets show high-level variability with amplitudes ranging from 0.25 to 1.1 mag, consisting of a periodic light variation onto which short-term fluctuations are superimposed. This variability pattern is very similar to the photometric behaviour of solar-mass, classical T Tauri stars. Low-resolution spectra of a few of these objects reveal strong Halpha and Ca-triplet emission, indicative of ongoing accretion processes. This suggests that 5-7% of our targets still possess a circumstellar disk. In combination with previous results for younger objects, this translates into a disk lifetime of 3-4 Myr, significantly shorter than for solar mass stars. The highly variable objects rotate on average slower than the low-amplitude variables, which is expected in terms of a disk-locking scenario. There is a trend towards faster rotation with decreasing mass, which might be caused by shortening of the disk lifetimes or attenuation of magnetic fields.Comment: 19 pages, 14 figures, A&A, in pres

The brown dwarf population in the Chamaeleon I cloud

We present the results of a multiband survey for brown dwarfs in the Chamaeleon I dark cloud with the Wide Field Imager (WFI) camera at the ESO/MPG 2.2-m telescope on La Silla (Chile). The survey has revealed a substantial population of brown dwarfs in this southern star forming region. Candidates were selected from R, I and H-alpha imaging observations. We also observed in two medium-band filters, M855 and M915, for the purpose of spectral type determination. The former filter covers a wavelength range containing spectral features characteristic of M-dwarfs, while the latter lies in a relatively featureless wavelength region for these late-type objects. A correlation was found between spectral type and (M855-M915) colour index for mid- to late M-type objects and early L-type dwarfs. With this method, we identify most of our object candidates as being of spectral type M5 or later. Our results show that there is no strong drop in the number of objects for the latest spectral types, hence brown dwarfs may be as abundant as low-mass stars in this region. Also, both kind of objects have a similar spatial distribution. We derive an index $\alpha = 0.6 \pm 0.1$ of the mass function in this region of dispersed star formation, in good agreement with the values obtained in other star forming regions and young clusters. Some of the brown dwarfs have strong H-alpha emission, suggesting mass accretion. For objects with published infrared photometry, we find that strong H-alpha emission is related to a mid-infrared excess, indicative of the existence of a circumstellar disk.Comment: Accepted for publication in Astronomy & Astrophysic